A radiation detector which includes a scintillator portion having a plurality of segments, and a light detection unit which detects scintillation light at a plurality of portions of the scintillator portion is known (for example, Japanese Unexamined Patent Publication No. 2007-93376, Japanese Patent No. 4332613, Japanese Patent No. 4338177, Japanese Patent No. 3597979, Japanese Patent No. 3697340, Japanese Patent No. 5013864, and WO 2012/105292 A). In the radiation detector, the segment which absorbs radiation is specified by the detection values of the scintillation light at the plurality of portions of the scintillator portion.